Abstract Peers are increasingly recognized as an important asset in healthcare. Peer support is historically most prominent for substance use disorder (SUD), such as the world-wide model of Alcoholics Anonymous and other 12-step groups. Yet for trauma and posttraumatic stress disorder (PTSD) there have been few attempts at peer help. A major challenge of trauma peer groups is that they usually focus on telling the story of members' trauma histories, which can be overly triggering and distressing. Seeking Safety offers an excellent choice for peer-led care. It is an evidence-based and is the most widely adopted model for SUD/PTD. It is present-focused, cognitive-behavioral, and provides psychoeducation and coping skills to help clients attain greater safety in their lives-- safety from unsafe behavior toward themselves and others, substance use, and trauma symptoms. It has been successfully implemented in peer-led format for many years and has been studied in various trials, including a recent randomized controlled trial (RCT) comparing peer versus professional delivery, with positive results. In Phase 1 we developed an exiting, beta version of a mobile app for peer-led Seeking Safety (PLSS). The Peer Safety (PS) app was based on extensive input from end users and was evaluated in a feasibility study. We found strong positive results in Phase 1 and now propose a Phase 2 project to continue this work. In Phase 2 we would enhance the app features and content and also conduct an RCT in a sample of adults with current SUD/PTSD to compare the PS app condition to a control app. They would have 12 weeks to use the app and participate in weekly online PLSS, and would be assessed pre, post and at 3-month followup using validated instruments. We will also measure app usage and satisfaction. The primary outcomes are substance use and emotion regulation. Our team offers an exceptional mix of expertise in SUD, PTSD, app development, clinical trials, Seeking Safety, peer services, and SBIRs. Our public health goal is to leverage technology to create PLSS in an online environment that would support recovery ?anytime, anywhere,? powered by highly engaging content and delivery. Most people with SUD/PTSD do not obtain formal treatment. The PS app could address the need for access in remote locations, for anonymous help, and for convenient, low cost help, delivered by people who share their experiences as peers. Even for those who do attend in-person modalities, the app can provide additional support and unique features. This project proposes a novel technology solution to provide care for an important population-- people with SUD/PTSD.